This invention relates to packless control valves used to control the flow of fluid in a piping conduit. A typical example of a packless valve that may be used for this purpose is a diaphragm type valve as illustrated, for example, in my U.S. Pat. No. 4,609,178. These valves provide good closure means and are reasonably compact. However, since the diaphragm acts both as a closure member and a stem seal, these diaphragms must be fairly large and are typically two to three times the diameter of the valve orifice. The result is a requirement for substantial actuating forces to overcome the forces created by the diaphragm area times fluid pressure. If the diaphragm is made out of a plastic, such as PTFE (Polytetrafluoroethylene), routine repeated closure of such a diaphragm is not guaranteed due to the high mechanical stress imposed upon by the large flexible movement required for such a diaphragm. Another disadvantage of a diaphragm valve is their highly streamlined flow passage which can lead to cavitation and wear under high fluid velocities. Finally, the body size of a typical diaphragm valve is fairly large in comparison to the port opening which makes the construction of such a valve fairly expensive. My invention overcomes these and other objections to valves of prior art by using a flexible tube as a closure element and stem sealing device. The flexible tube itself can be made from any commercial elastomeric or plastic, such as sanitary grade silicone rubber, without imposing undue stresses on the material. The relatively small amount of radial displacement of the sealing material requires only moderate actuating forces which leads to substantial cost savings as far as actuating devices are concerned. In comparison to diaphragm valves, which have relatively large areas subject to fluid pressure and, therefore, high stresses, my flexible tube is supported over more than 90% of the exposed area by an interiorly placed mechanical element resulting again in relatively low stress levels, thereby guaranteeing a substantially higher number of life cycles. Another advantage lies in the fact that my sealing tube diameter is only about 65% larger than the valve port diameter, therefore, requiring a relatively small valve housing and valve closure flanges. Finally, my invention provides for a valve that can be utilized in full vacuum without fear of the sealing membrane collapsing under a negative pressure gradient as is the case with typical diaphragm valves. These and other important advantages will be explained more clearly by the following drawings and descriptions.